Paper preprints are in decline in astronomy, while the use of electronic preprints is on the rise, mainly via the LANL astroph preprint archive. I discuss the decline of the paper preprint, some preprint servers available on the Web and some general electronic preprint issues, the astropharchive and its response to these issues, and alternatives for the future of electronic preprints.

The Astrophysics Data System (ADS) is an integral part of the Astronomy Digital Library and the collaboration Urania. It provides access to 1 million references and connects these references with many other information centers and their data, such as on-line journals, object databases, and scanned journal articles. This article describes some of the features and links between the ADS and other on-line services.

The Centre de Données astronomiques de Strasbourg (CDS) has dealt with bibliographic information for many years. References of publications, published observational data related to objects, data tables, nomenclature, have been homogenized and organized into information retrieval systems: SIMBAD, the reference database for the identification and bibliography of astronomical objects; the catalogue service and the VizieR catalogue Browser, for data tables; the Dictionary of Nomenclature of Astronomical objects, which is now maintained by the CDS. Evolution in recent years has brought the Data Centers closer to the publishing process. General standards for astronomy, for the description of references and of data tables, have been proposed and implemented. Data tables from papers are now directly published in electronic form, and distributed on-line by the Data Centers. The emergence of fully electronic publication paves the way to innovative new services, linking the journals to other sources of informations (data bases, tables, then images, data archives), and making use of new methods for information retrieval. This also has an effect on the publishing process, with the possibility to implement new checks and links from text to other kinds of information (from objects names, positions, etc.). The CDS will bring some of thekey features in the evolution towards a fully linked astronomy information system, in close collaboration with the journal editors, the ADS, the other Data Centers, and the data providers.

The developing countries at the low end of the scale face almost insurmountable problems in accessing electronic information. In Nigeria, only two of 40 universities have e-mail. Although electronic access to current information could revoutionize the educational situation at these places, the communication infrastructure does not exist, nor does the necessary funding.

The astrometric results of Hipparcos include the positions at epoch (1991.25), the proper motion in the new IAU extragalactic reference system (ICRS), and parallaxes for about 118 000 stars. One dimensional positions are also given for 48 asteroids and 3 satellites. Due to the non-isotropy of the scanning law, the uncertainties are position dependent. Some indications of the remaining correlations are given. Various tests and comparisons show that systematic errors in parallax, if any, are not larger than 0.1 millisecond of arc.

The Hipparcos Catalogue provides general astrometric and photometric information on double and multiple stars in specific fields of the main Catalogue and detailed data on the components in the various sections of a dedicated annex: the Double and Multiple Systems Annex (DMSA). Overall statistics of these solutions are presented for the 13211 entries of this annex and the different types of solutions are outlined.

During the development of the Hipparcos satellite, the opportunity to use the star mapper to perform a whole sky two colour intermediate accuracy photometric survey, and the main-mission detector for high accuracy photometry of target stars was identified. The instrument was optimised and the pre-launch calibrations performed to pre-determine the pass-bands with an accuracy sufficient to cope, in-orbit, with the foreseen aging of the detection chains. Photometry became an important by-product of the mission, very complementary to the astrometric results. It was the first opportunity to monitor the sky during 3-4 years without selection biases and with a precision similar to that achieved from the ground with the best classical techniques. A systematic detection of small amplitude variables was the expected return.

The Tycho Catalogue contains basic astrometric and photometric data for a million stars on the sky, complete to about V = 10.5 mag. The accuracy at the catalogue epoch (J1991.25) of positions and two-colour magnitudes is generally an order of magnitude better than has hitherto been available for the majority of these stars. Results from the production and verification of the catalogue is presented. The catalogue and associated epoch photometry annex are available in machine-readable form as part of the final Hipparcos mission products.

This contribution is intended as a ‘rough guide’ to the Hipparcos Catalogue for the non-expert user. We shall focus here on the interpretation of the measured parallaxes in terms of distances and luminosities of stars, and on the use of the covariance matrix of the errors on the astrometric parameters. Issues not addressed here are the level at which one may expect systematic errors to occur in the Hipparcos Catalogue, and the correlation of astrometric parameters over a given region of the sky. For more details on the latter two subjects we refer to the documentation of the Hipparcos Catalogue (ESA, 1997) and to Brown et al. (1997). We note that at present the Hipparcos Catalogue contains no identified systematic errors.

The Hipparcos astrometric parameters were obtained through the merging of abscissae data obtained by the FAST and NDAC Data Reduction Consortia. Using a different reduction scheme, other astrometric solutions may be obtained, although not statistically different: a weighted solution using FAST and NDAC astrometric solutions is shown, which gives both astrometric data and associated covariance matrix. This is applied to the acceleration and stochastic solutions. The intersection between merged and weighted solutions is then studied from the point of view of the possible signature of astrometric binaries.

New astrometric binaries were found during the Hipparcos data reduction process and their orbits were determined or improved. Other astrometric binaries may however be detected by searching for a quasi-sinusoidal signaturein right ascension and declination. For this purpose the observations of the candidate stars have been combined at a few different epochs, using the Hipparcos astrometric intermediate data.

A simultaneous fit of the astrometric and orbital parameters has then been done. However in most of the cases other ground-based data is needed in order to confirm the suspected binarity and to give a reliable orbit.

The ESA Hipparcos astrometry satellite did not contain an imaging detector (such as a CCD) but used a modulating grid of alternating transparent and opaque bands to modulate the light from a given object. The Fourier coefficients of the modulated detector signal were the basis for the astrometric and photometric processing leading to the Hipparcos Catalogue. These coefficients are included (for a subset of the stars) in the CD-ROM version of the Hipparcos Catalogue as the ‘Hipparcos Transit Data’. In this work, the Transit Data are used in a completely different and novel application: the modulation components from a single scan are considered to be equivalent to the interference fringes from an interferometer oriented along the scan direction and having baselines of 0, ≃ 10 and ≃ 20 cm and a wavelength of 550 nm. Using all the scans collected over the mission lifetime, it is possible to use the complex visibilities (containing bothamplitude and phase information) calculated from the Transit Data to createaperture synthesis images of various type of objects such as visual or orbiting binary systems. More information, including sample images, can be found in: Quist C.F., Lindegren L., Söderhjelm S., 1997, Using HipparcosTransit Data for Aperture Synthesis Imaging, Hipparcos Venice ‘97 Symposium, ESA SP-402 (in press).

Prof. M. Grenon asked the team of the General Catalogue of Variable Stars to attribute GCVS names to new Hipparcos variables in order to present, in the Hipparcos Catalogue, the information on these stars already with final designations. We received lists of 5665 variables. Of them, 3157 qualified to the standard GCVS criteria and will appear in the 74th Namelist of variable stars.

The main problems in compiling the Namelist were the following:

1. 1. Cases of spurious variability for double and multiple stars and for misidentified stars (wrong colour indices used in reductions). Some wrong identifications come from the SIMBAD data base.

2. 2. Lacking identifications with catalogues of spectroscopic binaries. Some stars attributed to slowly pulsating B variables are in fact ellipsoidal variables. HIP 98954 = V1472 Aql was classified as a semiregular star, but it is a rather unusual, probably eclipsing, star (Samus, 1997), a known spectroscopic-binary M giant with a 200d period (Lucke and Mayor, 1982).

The Second Cape Photographic Catalog (CPC2) contains 276,131 stars covering the entire Southern Hemisphere in a 4-fold overlap pattern. Its mean epoch is 1968, which makes it a key catalog for proper motions. A new reduction of the 5687 plates using on average 40 Hipparcos stars per plate has resulted in a vastly improved catalog with a positional accuracy of about 40 mas (median value) per coordinate, which comes very close to the measuring precision. In particular, for the first time systematic errors depending on magnitude and color can be solved unambiguously and have been removed from the catalog. In combination with the Tycho Catalogue (mean epoch 1991.25) and the upcoming U.S. Naval Observatory CCD Astrograph Catalog (UCAC) project proper motions better than 2 mas/yr can be obtained. This will lead to a vastly improved reference star catalog in the Southern Hemisphere for the final Astrographic Catalogue (AC) reductions, which will then provide propermotions for millions of stars when combined with new epoch data. These datathen will allow an uncompromised reduction of the southern Schmidt surveys on the International Celestial Reference System (ICRS).

We present a first attempt to compute an astrometric solution of POSS-IIplates using TYCHO stars as the reference catalogue. Also, we have made use of a special catalogue of stars observed with the Carlsberg Meridian Circlein La Palma between 1994 and 1995, brought into the HIPPARCOS reference system. Such a catalogue consists of a double strip along the equator, fairly dense (approximately 10 stars per square degree), faint (average magnitude 12.5), with an average error at epoch of ~ 150 mas per coordinate. Since the limiting magnitude of TYCHO stars is about 11.5, almost exactly one magnitude less than the CAMC special catalogues’ average, the latter has served as a very useful external comparison, although a more interesting magnitude range would have been 14-15. In this respect, we note that the Schmidt plates under study cover a large area (6.4 x 6.4 degree) and have fairly long exposures (~ 60 min); therefore, a possible magnitude dependence of the model needs to be carefully investigated. The results shown in the table above, which were obtained using a 2nd order polynomial in conjunction with a mask method using 5 plates only are very promising. The average external error over the entire plate area is about 0.2 arsecper coordinate, after the CAMC error have been subtracted.

We investigated the improvement of photographic astrometric observations of outer planets by use of Hipparcos Catalogue stars. The position of Jupiter on the plates was determined from measurements of the Galilean satellites, combined with ephemerides accurate to better than 10 mas. In a 6 contants model of photographic plates reduction, when images are near the center of the field, only offsets have to be computed, by considering the scale and orientation known. We applied this idea to the galilean satellites by using137 photographic plates realised between 1967 and 1974 by D. Pascu, with the Mc Cormick 26-inch refractor, Virginia, and its twin in the USNO, Washington DC. A study of the scale factors reveals an expected anisotropy of the scale factors. One possible hypothesis is that this effect is a consequence of the thermal expansions of the telescopic components. The use of Hipparcos data in the astrometric reduction allows to obtain an external error of around 40 mas on the determination of the observed absolute position of Jupiter. It is 5 times better than the classic computations using photographic or CCD reductions. Furthemore, with such results, badly-known effects can be easly estimated like the anisotropy of scale. At the end, important impacts on the fit of planetary ephemerides to the visual observations are expected.

The 3435 photographic plates, which were obtained by using the 40cm astrograph at Zô-Sè station of Shanghai Observatory during 1901-1993, have been accumulated for observing open clusters, extragalatic nebulas, radio stars and variable stars etc. These plates cover 764 unoverlapping regions (2°.3 * 1°.7) on the sky. Most of the plates distribute along the ecliptic, about 10 000 Hipparcos stars, 27 000 PPM stars and 500 FK5 stars are included in these regions. Among these plates, there are 2718 plates having two observing epochs, and they cover 359 different regions; there are 142 and 92 regions where the intervals of two epochs are about 50 and 70 years, and the numbers of plates in these regions are about1494 and 1023 respectively. About 372 plates were photographed before 1949,which play an important action for improvement of proper motions of Hipparcos stars. Four methods used to improve the proper motions for Hipparcos stars were designed. If the 2718 plates mentioned above having two epochs were used for improvement of the proper motions of Hipparcos stars, the stellar number of 299, 1378, 377 and 1664 corresponding to the four methods will be obtained with final precision 0.7 mas/yr. If supplementary observations are made in 1997, the stellar number of 494, 2431, 1534 and 2899 will be obtained with the same precision.

The external systematic accuracy of the HIPPARCOS positions and proper motions in the equatorial zone as represented by the subset of its bright FK5 stars is estimated by comparison with modern ground-based catalogues of 1980-1990s as well as with the most precise compiled catalogues made in the 20th century. Significant zonal and regional systematic differences between Hipparcos and ground-based catalogues have been detected for the different epochs ranging from 1920 to 1990. The most typical behaviour of the analysed differences at the epoch 1991.25 is almost absence of small-scale deviations and presence of the large-scale features. For the epoch 1920, both large and small-scale discrepancies are more pronounced reaching up to 50 mas. This gives evidence of the inferiour consistency of the HIPPARCOS system with ground-based observations comparing with the best compiled catalogues.